I should note, that OpenWatcom 2.0[1] is far better for supporting more recent C and C++ code, modern hosts and tooling, but still able to compile into 16 bit code. It is also actively maintained. Instead of MASM I recommend JWasm[2] + Jwlink[3]. Back in time I did a fork[4] of JWasm that has cleaner build system (CMake).
I clicked on the first GitHub link and landed on the repo page for OpenWatcom. Immediately I hit one of my biggest pet peeves with GitHub repo pages: what is it? The readme doesn’t say!
Repo maintainers: please put a one-or-two sentence description of what your project is at the top of the repo readme! It’s really not too much at all to ask! Assuming people know what your project is just because they landed on the repo page is one of the fastest ways to bounce people out.
[1] https://github.com/open-watcom/open-watcom-v2
[2] https://github.com/Baron-von-Riedesel/JWasm
[3] https://github.com/JWasm/JWlink
[4] https://github.com/JWasm/JWasm